1. Field of the Invention
The invention relates to resonant filters for improving adjacent channel selectivity.
2. Description of the Related Art
Spikes with slight attenuation behavior that negatively influence the adjacent channel selectivity in a particular frequency range are usually found at the left side of the pass-band in the transfer function of Dual Mode Surface acoustic wave SAW (DMS) filters. When, for example, such a filter is utilized as an input filter in the front end of a mobile radiotelephone device, then the disturbing spikes in the transfer function lie in the region of the transmit (“TX”) band that is provided for the incoming signals. The TX suppression is unacceptably slight due to the spikes in the TX region, particularly given single-track DMS filters, so that the filter cannot be utilized as a TX filter.
For solving this problem, DMS filters with a plurality of cascaded tracks were previously utilized, where these tracks are differently designed in terms of one or more parameters. In particular, DMS filters are utilized in which two filter tracks are cascaded whose finger periods of the inter-digital transducers are differently selected. It is also possible that the tracks of cascaded DMS filters differ in terms of other parameters, this allowing the spikes at the left side of the pass-band to be successfully reduced. The desired effect, however, is not adequate and, in particular, a satisfactory possibility of eliminating these disturbing spikes has not yet been found for one-track DMS filters.